This invention relates to an image forming apparatus which forms images by superposing a photosensitive sheet with one surface coated with microcapsules encapsulating a colorless dye and a light-curable agent which hardens by exposure to light and an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on this colorless dye and pressing them together.
Japanese Patent Publication Tokkai 58-88739 discloses a method of forming images by using a photosensitive sheet. The photosensitive sheet used by this method is characterized as having a sheet-like light-transmissive base material, for example, of polyester coated with microcapsules encapsulating a colorless dye of a specified kind and a light-curable agent which hardens when exposed to light. If a photosensitive sheet like this is exposed to image-forming light such as reflected light from a document, only those of the microcapsules on the sheet which are exposed to light are hardened and a latent image corresponding to the image carried by the light is thereby formed on the photosensitive sheet. If an image transfer sheet with one surface of its base sheet coated with a developing agent having a chromogenic effect on the colorless dye in the microcapsules is superposed on such a photosensitive sheet having a latent image formed thereon and if they are then pressed together, those of the microcapsules which have not been hardened rupture, causing the colorless dye encapsulated therein to flow out thereof and to react with the developing agent on the image transfer sheet to form a visible image corresponding to the latent image. Examples of the base sheet for an image transfer sheet include thin sheets of unit weight less than about 90 g/m.sup.2, thick sheets of unit weight greater than about 120 g/m.sup.2 and light-transmissive sheets which can serve as originals for an overhead projector (OHP).
In order to rupture the microcapsules which have not been hardened on a photosensitive sheet described above, a high pressure on the order of 25-125 kg/cm is required and use is usually made of a compressing device with a pair of pressure rollers for applying such a high pressure. Such compression devices have been disclosed, for example, in U.S. Pat. No. 4,399,209 and Japanese Patent Publication Tokkai 62-6257.
When a photosensitive sheet and an image transfer sheet are superposed and pressed together to rupture those of the microcapsules on the photosensitive sheet which have not been hardened, the resultant color image may have different density from what is expected if pressure is not applied stably and the microcapsules are not ruptured dependably. Even if the same compression device is used, density may be low if the image transfer sheet is thin while density may be high if a thick image transfer sheet is used. Japanese Patent Publications Tokkai 62-209535 and 62-209536, for example, have disclosed methods of controlling the rotational speed of the pressure rollers and the magnitude of applied pressure according to the density of the original, image density inputted by means of density selection switches, etc. With these methods, however, changes cannot be effected instantly corresponding to the changes in the condition of compression due to changes in the thickness of the image transfer sheet, which is usually determined by the kind of its base sheet.
For the purpose of providing lustre to the image formed on the image transfer sheet, it is known to preliminarily coat the sheet with a thermoplastic resin material and to heat the sheet after a visible image is transferred thereonto, thereby melting and softening the surface-coating resin and covering the visible image therewith. Examples of heating method for thus providing lustre to an image transfer sheet include the oven method whereby the image transfer sheet with an image formed thereon and being transported is indirectly heated from above by a heater, the heat roller method whereby the image transfer sheet with an image formed thereon is passed between a heat roller enclosing therein a heat source such as a halogen lamp and a pressure roller compressed against the heat roller, and the belt method whereby heating is effected by means of a heat-resistant belt wound around a similar heat roller enclosing therein a heat source such as a halogen lamp.
As explained above, however, the thickness of the image transfer sheet is not constant because base sheets of various thicknesses are used and light-transmissive sheets capable of serving as originals for an overhead projector (OHP) must also be accommodated. Moreover, the proper amount of heat required to give lustre to the image transfer sheet depends on its material characteristics as well as its thickness. If the heating is effected at the same temperature by any of the methods described above, the resultant lustre becomes uneven in the case of paper sheets and the light transmissivity becomes non-uniform in the case of original transparencies for an overhead projector (OHP).